ABSTRACT This project seeks to understand whether, for whom, and how the effects of successful early childhood school readiness interventions are sustained across a child?s development. Past seemingly contradictory findings about so-called fadeout motivate this project. Our first aim will assess the long-run effects of two early childhood interventions implemented at scale in North Carolina. We will estimate the treatment impacts of the More at Four (MF) pre-kindergarten (pre-k) intervention program and the Smart Start (SS) Initiative on key health, education, and well-being outcomes during adolescence (e.g., high school graduation, teen pregnancy). MF is a high-quality pre-k program for low-income children, measured at the individual level by enrollment and at the community level as funding allocations per capita by the state to each of 100 counties in each of 15 years. This funding was allocated to over one million children. Gradual rollout of MF and quasi-random variations led to sizeable differences in funding across years and counties that are independent of child characteristics. SS provides funding to counties to improve quality and access to child care, health screenings, and other services for all children from birth to age 5. SS was implemented using the same gradual rollout and funding formula used for MF, but in different years, providing independent random variation in program access. As we test long-run impact, we will also test whether effect sizes vary as a function of family income, ethnicity, dual-language learning status, and other characteristics. Our second aim will test the hypothesis that the impact effect size varies with the school environment that a child enters, called the Sustaining Environment hypothesis. We hypothesize that children who experience a school readiness intervention and are then placed into a sustaining environment (e.g., classroom with higher proportion of peers who are ready to learn or have experienced the early intervention, classroom teacher who has been trained to promote learning) will have longer-lasting effects (a moderation effect). The third aim is to test the Sustaining Environment hypothesis through experimental manipulation in the Building Blocks (BB) mathematics intervention study. The BB pre-k math curriculum focuses on numeracy and geometric/spatial reasoning. In 2005, 42 schools in Boston and Buffalo with 1,375 children were randomly assigned to one of three conditions: (1) BB; (2) BB plus elementary school teacher training to support persistence of early impact; or (3) control (pre-k as usual). We will add measures of the child's subsequent peer, classroom, and school environments, as well as adolescent outcomes, to test the hypothesis that the BB program has longer-lasting effects when implemented with a subsequent sustaining environment. The final aim is to integrate findings across studies to reach generalizable conclusions about the circumstances in which early intervention has impacts that persist or fade across development, for both development scientists and education policy makers.